Silicon carbide (SiC) is a desired material for use in the manufacture of semiconductor devices. Silicon carbide has a variety of properties useful in such devices, including a wide bandgap, a high thermal coefficient, a low dielectric constant and high temperature stability. As a result, silicon carbide materials can provide excellent semiconducting properties, and electronic devices made from silicon carbide can perform at higher temperatures as compared to devices made from other materials currently used in the industry.
Growing relatively large silicon carbide crystals, from which a large number of wafers may be produced, can be economically advantageous. Additionally, many semiconductor applications require a single crystal material with very low concentrations of intrinsic point defects in the crystal lattice and/or very low concentrations of unwanted impurities. Even in a pure material, a defective lattice structure can prevent the material from being useful for electrical devices. Certain intrinsic point defects significantly impact important fundamental properties of semiconductors crystals such as density of deep levels, minority carrier life time, carriers mobility, concentration of active donors and acceptors, dislocation distribution, stacking fault energy formation, local band gap variation, and the like.
Traditionally, two broad categories of techniques have been used for forming crystalline silicon carbide for semiconductor applications. The first of these techniques is known as chemical vapor deposition (“CVD”) in which reactant gases are introduced into a system to form silicon carbide crystals upon an appropriate substrate.
The second main technique is generally referred to as sublimation. In this technique, a solid silicon carbide source material (typically a powder) is used as a starting material. The solid silicon carbide starting material is heated in a crucible until it sublimes. In particular, at process temperatures of about 1800-2400° C., the volatile species Si, Si2C, and SiC2 sublime from the silicon carbide source material.
The sublimed material is encouraged to condense to produce the desired crystal. This can be accomplished by introducing a silicon carbide seed crystal and SiC powder into the crucible and heating it to a temperature at which silicon carbide powder sublimes. A pioneering patent that describes methods for forming crystalline silicon carbide for semiconductor applications using such seeded sublimation techniques is U.S. Pat. No. 4,866,005 to Davis et al., issued Sep. 12, 1989, which was reissued as U.S. Pat. No. Re. 34,861, issued Feb. 14, 1995, which patents are incorporated herein by reference as if set forth in their entirety.
Despite advancements in silicon carbide crystal growth techniques, it can be difficult to control various process parameters to minimize intrinsic point defect concentrations in the resultant silicon carbide crystal. As an example, the stoichiometry of the vapor phases present within a crucible can influence crystal growth and intrinsic point defect generation. At all temperatures, the gas phase is rich in the silicon component. Conventionally, conditions within the crucible are selected to provide a Si:C vapor phase ratio that is significantly above unity to avoid graphitization and other defect-generating reactions at the growing crystal surface. See pages 16-17 of Process Technology for Silicon Carbide Devices, edited by Carl-Mikael Zetterling, KTH, Royal Institute of Technology, Sweden (2002).
A very high Si:C vapor phase ratio, however, generates a very high concentration of carbon vacancies in the growing SiC crystal. Point defects/complexes associated with the carbon vacancies (Z1Z2 and EH6/7) are very efficient deep recombination centers in SiC and therefore are minority carrier lifetime-limiting point defects in SiC crystals and epitaxial layers. For example, by reducing the Si:C gas phase ratio during the CVD growth of SiC epitaxial layers, it is possible to significantly reduce the concentration of Z1Z2 and EH6/7 point defects/centers in the SiC epitaxial layers and, therefore, significantly increase minority carrier lifetime. J. Zhang et al., J. Applied Physics, Vol. 93, No. 8, pages 4708-4714 (2003) and Y. Negoro et al., Applied Physics Letters, Vol. 85, No. 10, pages 1716-1718 (2004). A high minority carrier lifetime in the SiC crystal and/or epitaxial layer is very important for the design of high power SiC devices.
Sublimation processes are conventionally conducted in a closed crucible. Conducting the sublimation process in a closed crucible prevents the Si-rich gas phase from leaving the crucible and thus can aid in maintaining the Si:C ratio significantly above unity. While useful, this also provides only limited process control and further does not readily allow adjustments to the Si:C ratio as necessary during sublimation.
Therefore, a need exists to better control the stoichiometry of vapor phases within a sublimation chamber during a seeded sublimation process to minimize intrinsic point defect generation in the resultant bulk silicon carbide single crystal.